One of the most challenging aspects of controller area network (CAN) transceiver design is the development of a driver output that meets the stringent electromagnetic compatibility (EMC) standards required by customers. A typical CAN transceiver consists of a CAN driver and a CAN receiver that are internally connected in the integrated circuit by respective common pins, CAN high (CANH) and CAN low (CANL). The driver portion connected to the CANH pin has only an active pull-up device and the driver portion connected to the CANL pin only has an active pull-down device. The ISO 11898 physical layer standard allows CANH to be actively pulled high, but must be passively pulled low by an external load or light internal biasing provided by the CAN receiver. The CANL pin is just the opposite. CANL can be actively pulled low, but must be passively pulled high by the external load or light internal biasing provided by the CAN receiver. This physical layer standard was chosen to avoid driver arbitration issues when one driver on the bus is trying to pull a bus line low while another driver is trying to pull the same bus line high.
The driver outputs turn on into a low impedance state called the dominant state in CAN nomenclature. This is commonly referred to as a digital “1” in a positive logic system. The driver outputs turn off into a high impedance state called the recessive state in CAN nomenclature. This is commonly referred to as a digital “0” in a positive logic system. The recessive state is also considered to be a tri-state condition with a light bus biasing. The typical DC load on a CAN driver is a 60 Ω differential load as specified by the standard. A typical CAN receiver consists primarily of a resistor attenuator and a hysteresis comparator. The attenuator is necessary to attenuate the external common-mode of the bus signals on CANH and CANL down to values that are within the power supplies of the CAN transceiver. The common-mode range of the ISO11898 standard is specified as a minimum of −2 V to +7 V while the power supply is nominally 5 V and 0 V. High performance CAN transceivers have stretched the allowable bus common-mode range to −12 V to +12 V and higher. Thus, the attenuator must be designed to conform to the ISO11898 standard.
Many applications have stringent electromagnetic compatibility requirements for CAN transceivers. The manner in which the ISO11898 CAN physical layer is specified leads to major difficulties in designing balanced CAN driver outputs that minimize emitted radiation. Since the CANH pin can only pull actively high and the CANL pin can only pull actively low, the driver design is especially difficult to balance out when transitioning from a dominant state to a recessive state.